Melatonin is a naturally occurring hormone in the human body. It is made in the brain by the pineal gland. It can also be made synthetically in a laboratory. Melatonin plays an important role in sleep and wake cycles. People take this supplement to help adjust their body's internal clock. Blind people use melatonin to help establish their day and night circadian rhythms, as it can be difficult to do without any light input during the day.

Other people take melatonin to help decrease their jet lag, to help adjust to schedule changes due to shift work, and to help combat insomnia. Some people even use melatonin to reduce the side effects of nicotine withdrawal when they are attempting to stop smoking. In some cases melatonin is given to calm patients before they are given anesthesia prior to surgery. Melatonin can even be used in children to help establish better sleep wake cycles.